Process of desalting pickled hides and skins



patented July 25 195 UNITED STATES OF FlCE EPROCESS OF DESALTING-PICKLED HIDES "AND SKINS Charles GLSchneider, Brentwoo'd Borough, Pa., assignor to Hall Laboratories, Inc.

No Drawing. Application March 12, 1947,

. ..Serial No. 734,226

, .fIhis invention relates to the desalting of pick- .led hides and skinswith very dilute solutions of alkali-metal, metaphosphates.

The purpose of this, treatment is to remove most of the sodium chloride from thepickled stock while preventing or materially reducing the acid-swelling and gelatinization of the stock.

Pickling is a well known operation which has been. practiced in. the tanning industry for many years. Bated hides and skins generally do not .possess the desired uniformity of condition required for proper-tanning. The pickling process .has been found an effective method of bringing about.tl1is.desireduniformity. At the. same time it also assists in the removal of stains from the stock. Although .any stock many be pickled, the

. operation is generally confined to the lighter types such assheepskins, goatskins, and calf- ..skins. John Arthur Wilson in his treatise Mod;

. ern. Practice in Leather Manufacture, published by Reinhold Publishing Company in 1941, de-

.scribes the pickling of bated calfskins on pages 265-267 in the following manner:

Dissolve 2500 lbs. of salt (sodium chloride) in water to make a totalvolume of 1900 gallons of brine at- 60 F. in a 2500 gal. paddle vat, and add 95 lbs. of concentrated sulfuric acid (66 Baum).

. Mix well, start the paddle running and dump 5000 .lbs. of bated caliskins into the liquor.

ning for. one hour,..take a sample of the pickle liquor for analysis. analyzing the salt and acid content of the:solu

. .tion arethen given.)

(Details of procedurelior After the paddle has run two. hours, againdetermine the amounts of salt .and acid. .Leavethe stock in the liquor over night, having the, paddle. run for about one minute each hour, if possible. Next morning, run the paddle for thirty minutes, take a sample, and add whatever may be necessary to make a final concentration of one pound or salt and 0.01 pound of acid-per gallon. Then haul out the stock and pile it on wooden horses or on a fiat truck.

Pickled skins can be vegetable-tanned in approximately the same manner as bated unpickled skins. "However, certain precautions must be taken or the stock can be easily ruined. The only purpose of the salt in pickling is to prevent inordinate acid-swelling and gelatinization of the stock; if pickled skins are suspended inordinary vegetable tanning liquor, the salt rapidly difiuses 7 Claims. (Cl. 894.15)

2 out of themv and into the tanning liquor. Since the liquor is not sufficiently saline to check the swelling reaction, the skins could easily become a total loss in a few hours. Hcient salt is dissolved in the first tanning solution Consequently, suinization must be carried out in a solution of salt which is'sufficiently concentrated to prevent acid swelling. After the pH value of the solution in equilibrium'iwith the stock is above the iso-elecsalt.

'tricpoint of" the hide protein, the stock. may be washed in running water without damage. Such de-pickled stock may then be vegetable tanned or otherwise treated in salt-free liquors.

For some purposes it is desirable to treat hides or skins in acid solution containing little or no An example of this type of treatment is the tanning method disclosed in the Wilson patents noted below. When pickled skins areto be subjected to such a process, a substantial portion of the salt should first be removed from the skins.

After. run- I As the tanning operation is performed in acid solution, no appreciable change should be made 7 in the acid condition of the stock while the salt is being removed. By, practicing my invention, it is possible toremove enough salt from the pickled skins to permit certain further. processing without the. necessity of removing. acid as by the neutralizing process commonly employed.

For the purpose. of describing this invention, the term desalting is intended to meana process of drumming pickled skins in an aqueous solution of certain phosphates, whereby asubstantial amount of the salt in the skins is removed without any appreciable effect on the acid content of the skins.

While my process may be employed in treating .all types of pickled stock regardless of the tannage to follow, it is most feasibly adapted to the removal oflsalt fromskins which are to be tanned or pre-tanned with a molecularly dehydrated phosphate, This type of tannage is disclosed in several patents. including U. S. Patents 2.14:0,0tl

and 2,140,042, issued to John Arthur Wilson, as well as U. S. Patent 2,140,008, issued to Ralph E. Hall. The molecularly dehydrated phosphates are usually employed as a pro-tannage in con junction with other tanning processes.

I have discovered that at a pH value of 2.2-2.5, which is the optimum for tanning with the so-called sodium metaphosphate or Grahams salt having a ratio of NazO:PzOs of 1:1, or a sodium phosphate glass having a ratio of N azO :P2O5 of 1.16:1, it is not possible to tan pickled and unpickled stock with an equal degree of success. This is unquestionably due to the presence of salt in the stock. I have found that the amount of phosphate which will combine with hide is decreased as the salt concentration of the phosphate tanning solution is increased. For instance, if the phosphate solution contains 4% of sodium chloride, a hide tanned in the solution will combine with only half as much phosphate as it would if the salt were absent. Although it is possible to pre-tan salted pickled stock with glassy sodium phosphates, an excessively high concentration of phosphate is required to overcome the effect of he salt carried into the tanning solution by the pickled stock. For example, in Wilson Patent 2,140,042, 40 pounds of Graham's salt is added to a solution used for drumming 1000 pounds of bated skins. I have found that when 40 pounds of the same phosphate is added to pickled stock of the same type considerably less actual tanning occurs. In fact, to achieve an equal tanning effeet, from 80-100 pounds of phosphate would be required for a pickled stock of similar origin.

Surprisingly, I have found that when I dissolve from about 5 to pounds of this particular phosphate in a quantity of water ranging from 200%-500% of the weight of the 1000 pounds of pickled stock, I am able to remove enough of the unwanted salt by drumming for an hour or less to permit subsequent tannage with about 40 pounds of sodium metaphosphate. In other words, by desalting 1000 pounds of pickled stock with not over 10 pounds of metaphosphate, followed by a pretannage with not over 40 pounds of the same phosphate, I achieve a result with 50 pounds of phosphate in a two-stage operation which could not be achieved with much less than 100 pounds of phosphate if my desalting operation were omitted.

In practicing my invention, I prefer to use about 0.5% of a sodium phosphate glass having a ratio of NazOtPzOs of 1.16:1, based on the weight of the pickled stock. I dissolve the phosphate in a quantity of water which may Vary from about 200% to about 600% of the weight of the stock. If less than 200% water is used, the concentration of salt in the desalting solution is so great that when the stock is drained, an appreciable amount will be carried over into the metaphosphate tanning operation where it interferes with optimum tanning. If more than 600% water is used, the extreme dilution of the salt as well as the corresponding dilution of the phosphate causes the stock to swell. Obviously, it is more efficient to desalt with a minimum of water since more stock can be treated in each drum. However, the most preferable amount of water would appear to be somewhere between 200%- 600% of the weight of the stock.

to evaluate this method of desalting pickled stock. In each of six bottles I placed a piece of pickled sheepskin and 0.5% by weight of a sodium phosphate glass having a ratio of NazO'zPzOs of I have performed a series of laboratory tests 1.16:1. Water was added in amounts varying from 200%-100094, of the weight of the sheepskin and the bottles were shaken for one hour. The first four samples desalted satisfactorily without swelling. Sample No. 5 showed signs of slight swelling while sample No. 6 was badly swollen.

Table Per Cent Per Cent Per Cent P 0 Sample Water gg combined pH glue with skin 1 200 5. 98 0. 05 1.35 300 4. 50 0. 12 l. 50 400 3. 41 0. 17 l. 60 4 500 2. 79 0. 34 l. 70 5 600 2. 39 0. 31 1. 80 6 l, 000 1. 44 O. 91 2. 00

In a control set of samples without phosphate, only the sample run with 200% water did not swell, the concentration of salt in the solution being high enough in this case to repress swelling.

I may use less than the preferred 0.5% phosphate concentration and in some instances may employ more than this amount. Generally speaking, from 0.10%-1% based on the weight of the stock is an acceptable range, depending upon the type of stock, the degree of pickle, and other factors. I have found that as little as 0.05% of phosphate represses swelling if about of water is employed. However, to obtain more eiiicient removal of the salt, it is preferable to use larger percentages of water. Under these circumstances, it is of course necessary to use a somewhat higher concentration of phosphate. The phosphate may be thrown dry into the drum at the time it is loaded, although it is preferable to prepare a solution to insure rapid dilution in the drum and to avoid swelling of the stock. Drumming in this dilute solution is carried out for a varying length of time, again depending upon the degree to which the skins are pickled and the type of stock being treated. Generally speaking, the drumming operation will'require from 15 minutes up to several hours, after which time the stock is drained and made ready for tanning. The stock should not be washed with water before tanning since the combination of P205 with hide substance in the desalting operation is of such a small magnitude that immediate swelling would occur if the salt were virtually all removed.

In practicing my invention, I propose to use the alkali-metal or ammonium salts of phosphoric acids which are capable of coagulating albumen. These salts may be designated as alkali-metal molecularly dehydrated phosphates having a ratio of alkali-metal oxide to phosphorus pentoxide of about 0.4:1 to 1.7:1. Such salts include the molecularly dehydrated crystalline ammonium phosphate having a ratio of (mm 501F205 of about 1:1, and are characterized by their ability to coagulate albumen. I have found that these salts appear to combine with the protein or hide substance to a sufficient extent that swelling of the stock is prevented. Salts of those phosphoric acids which are not capable of coagulating albumen do not seem to thus combine and consequently do not repress the swelling of the skins.

I prefer to use sodium phosphate glasses having a ratio of Na2O:PzO5"of from 1.121 to about 1.25:1in my desalting process because they are 3 readily. .iobtainable at... reasonable .cost: Since aqueous solutions o'f these phosphates have-pH values of from about 6.5 to about 8, they .have onlyaslight neutralizing effect on the acid present in the desalting solution. For example, when I drummed pickled calfskin with 200% of water, the p I-I'value of the solution was 2.05, but when '.5% of a sodium phosphate glass having a ratio of 1.16 N azOzl P was added, the pH value was 2.10, indicating that' there was practically no neutralization of the acid by the phosphate addition. "However, I may 'use other molecularly dehydrated phosphates and achieve satisfactory results, provided the phosphate is the salt ,of. an acid which is capable of coagulating albumen. For examplejthe' 'so-called sodium tetraphosphate,'NasP4O13,"having a ratio .or'Na20:P2o5 of about 1.521 is an effective salt. An aqueous solo.- tion of this salt has a pH value of about 8.2. Broadly speaking, molecularly dehydrated phosphates having a ratio of alkali metal oxide to phosphorus pentoxide from about 0.4:1 to about 1.7 1 may be employed. Excluded are those salts derived from acids incapable of coagulating albumen. An example of this inoperative group is sodium trimetaphosphate (NaPO3)3, derived from trimetaphosphoric acid (HPO3)3, which is incapable of coagulating albumen. The pyrophosphates and orthophosphates, derived from pyrophosphoric and orthophosphoric acid respectively, are ineffectual. These acids will not coagulate albumen.

It should be pointed out that many compounds will exert what is commonly known as a depickling effect as contrasted with a de-salting effect. Depickling is nothing more than the neutralization of the acidic solution with alkaline materials such as caustic soda, soda ash, sodium bicarbonate, and other alkalies. By adding alkali, the pH value of the solution is increased and the swelling tendency of the stock is reduced. However, the addition of alkaline materials not only causes the formation of unwanted salts in the solution but also necessitates the subsequent use of further quantities of acid to restore the pH value of the stock to the comparatively low level required for tanning. Therefore, it is obvious that the preferred material to be used is one which causes little or no increase in the pH value of the desalting solution.

There are certain phosphates which depickle but do not desalt. As has been shown, the alkaline orthoand pyrophosphates will not repress the inordinate swelling of pickled stock if the pH value of the solution is maintained at a constant level by additions of acid. However, since these compounds are quite alkaline, they would function as depickling agents if added to the solution without attempting to control the pH value. It is not within the scope of my process to claim the obviously old depickling method by means of neutralization, but it should be pointed out that there are alkaline-reacting phosphates which repress the swelling of hides by a fixation of hide substance, and I do not confine my process to the use of only those phosphates which are neutral or acidic in nature. Typical of alkaline-reacting phosphates which repress acid swelling by the partial combination with hide substance are the tripolyphosphates such as crystalline sodium tripolyphosphate Na5P3010 and the phosphate glasses having the nominal composition of tetraphosphate, for example NasP4O1s.

The various specific compounds set forth are given only by way of example to illustrate the more common alkali metal molecularly dehydrated phosphatesincluding metaphosphoric acid. The correspondingpotassium, lithium, and amnionium'saltsimayalso be used. I have 'found crystalline".ammonium metaphosphate quite satisfactory 'for:=-this.purpose. In addition to the water-soluble salts, I may employ such derivatives of metaphospho'ric :acid as the water-insoluble crystalline sodiumzmetaphosphate and the corresponding. potassium salt; since these compounds are solublerin'theacidic solution used in the practice'of myprocess Havingfthus' described'my invention, what I claim as new and desire to secu'rebyLetters Patent'is: w. V

1; Theamethod'of de sa'ltin'g pickled stock, such asihid'es andskinsand avoiding inordinate acidswellingand gelatinization thereof, which comprises drumming the salted pickled stock in an aqueous solution containing a weight of water which is two to six times that of the pickled stock, said solution having dissolved therein a salt of a phosphoric acid capable of coagulating albumen, said salt being selected from the group consisting of alkali-metal salts having a ratio of alkalimetal oxide to phosphorus pentoxide of from about 0.421 to about 1.7:1, and the ammonium salt crystalline ammonium phosphate having a ratio of (NHQZO to phosphorus pentoxide of about 1:1, and being present in an amount which is from about 0.05% to about 1.0% based on the weight of the pickled stock.

2. The method of de-salting pickled stock, such as hides and skins whereby inordinate acid-swelling and gelatinization is prevented, which comprises drumming the salted pickled stock in an aqueous solution containing crystalline ammonium phosphate capable of coagulating albumen and having a ratio of (NI-L920 to phosphorus pentoxide of about 1:1, the concentration of said phosphate in said solution being from about 0.05 to about 1% of the pickled weight of the stock, and the water content of said solution being about of the weight of said stock.

3. The method of de-salting pickled stock, such as hides and skins which comprises drumming the salted pickled stock in an aqueous solution containing about 0.05% to about 1% based on the weight of the stock, of an alkali-metal molecularly dehydrated phosphate capable of coagulating albumen and having a ratio of alkali metal oxide to phosphorus pentoxide of from about 0.4:1 to about 1.721, said phosphate being a salt of a phosphoric acid capable of coagulating albumen, the amount of water in said solution being from 100% to about 600% based on the weight of the stock, said phosphate and water concentrations acting to prevent acid-swelling and gelatinization of the stock while removing salt but not the acid from the stock.

4. The method according to claim 3 characterized by the fact that the alkali-metal molecularly dehydrated phosphate is a sodium phosphate.

5. The method of de-salting pickled stock, such as hides and skins whereby inordinate acid-swelling and gelatinization is prevented, which comprises drumming the salted pickled stock in an aqueous solution containing a molecularly dehydrated alkali-metal phosphate having a ratio of alkali-metal oxide to phosphorus pentoxide of from about 0.421 to about 1.7 :1, said phosphate being a salt of a phosphoric acid capable of coagulating albumen, the concentration of said phosphate in said solution being from about 0.05

76 to about 1% of the pickled weight of the stock.

and the water content of said solution being about 100% of the weight of said stock.

6. The method according to claim 5 characterized by the fact that the molecularly dehydrated alkali-metal phosphate is a sodium phosphate glass having a ratio of NazOzPzOs of from about 0.921 to about 15:1.

7. The method of de-salting pickled stock, such as hides and skins and avoiding inordinate acidswelling and gelatinization thereof, which .comprises drumming the salted pickled stock in an aqueous solution containing a weight of water which is two to six times that of the pickled stock, said solution having dissolved therein, an alkali metal salt of a phosphoric acid capable of coagulating albumen, said salt'having a ratio of alkali metal oxide to phosphorus pentoxide of from about 0.4: 1 to about 1.7: 1, and being present in an REFERENCES CITED The following references are of record in the file of this patent:

FOREIGN PATENTS Number Country Date 478,773 Great Britain Jan. 25, 1938 481,635 Great Britain Mar. 15, 1938 484,781 Great Britain May 10, 1938 OTHER REFERENCES- Ser. No. 374,084, Riess (A. P. 0.), pub. May 11, 1943. 

1. THE METHOD OF DE-SALTING PICKLED STOCK, SUCH AS HIDES AND SKINS AND AVOIDING INORDINATE ACIDSWELLING AND GELATINIZATION THEREOF, WHICH COMPRISES DRUMMING THE SALTED PICKLED STOCK IN AN AQUEOUS SOLUTION CONTAINING A WEIGHT OF WATER WHICH IS TWO TO SIX TIMES THAT OF THE PICKLED STOCK, SAID SOLUTION HAVING DISSOLVED THEREIN A SALT OF A PHOSPHORIC ACID CAPABLE OF AOAGULATING ALBUMEN, SAID SALT BEING SELECTED FROM THE GROUP CONSISTING OF ALKALI-METAL SALTS HAVING A RATIO OF ALKALIMETAL OXIDE TO PHOSPHORUS PENTOXIDE OF FROM ABOUT 0.4:1 TO ABOUT 1.7:1, AND THE AMMONIUM SALT CRYSTALLINE AMMONIUM PHOSPHATE HAVING A RATIO OF (NH4)2O TO PHOSPHORUS PENTOXIDE OF ABOUT 1:1, AND BEING PRESENT IN AN AMOUNT WHICH IS FROM ABOUT 0.05% TO ABOUT 1.0% BASED ON THE WEIGHT OF THE PICKLED STOCK. 